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Structured logging

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The @std/log package routes log records through configurable handlers with severity levels. This example sends INFO and above to the console, writes WARN and above to a file as JSON lines, and shows that records below a handler level are filtered out.

import * as log from "jsr:@std/log";

const logFile = "/tmp/structured_logging_example.log";
A handler decides where records go and which levels it accepts. The formatter option customizes the output line, here as JSON without timestamps to keep the example deterministic.
const fileHandler = new log.FileHandler("WARN", {
  filename: logFile,
  formatter: (record) =>
    JSON.stringify({ level: record.levelName, message: record.msg }),
});
Setup wires handlers to the default logger. The console handler only accepts INFO and above, and useColors false keeps the output plain.
log.setup({
  handlers: {
    console: new log.ConsoleHandler("INFO", { useColors: false }),
    file: fileHandler,
  },
  loggers: {
    default: { level: "DEBUG", handlers: ["console", "file"] },
  },
});
This record is below INFO, so neither handler prints or stores it.
log.debug("cache miss for key user:42");
INFO reaches the console handler but not the WARN file handler.
log.info("server listening on port 8000"); // INFO server listening on port 8000
WARN and ERROR reach both handlers.
log.warn("disk usage at 81%"); // WARN disk usage at 81%
log.error("upstream timed out"); // ERROR upstream timed out
The file handler buffers writes, so flush before reading the file.
fileHandler.flush();

const contents = await Deno.readTextFile(logFile);
console.log(contents.trimEnd()); // {"level":"WARN","message":"disk usage at 81%"}
{"level":"ERROR","message":"upstream timed out"}
Close the handler and remove the file to clean up.
fileHandler.destroy();
await Deno.remove(logFile);

Run this example locally using the Deno CLI:

deno run -R -W https://docs.deno.com/examples/scripts/structured_logging.ts

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